1. Field of Invention
The invention relates to a web-guiding or sheet-guiding machine and a method of operating such a machine. Web-guiding or sheet-guiding machines are used in extremely diverse areas of technology. These machines usually include printing machines and laminators. Machines of this type likewise include those used for winding or unwinding webs and oscillating units. Film extrusion systems also have film-transport rollers and squeeze rollers, which are upstream of the winding devices, if any.
2. Description of the Prior Art
All these machines have a plurality of transport rollers and often also roller nips. The term “roller nip” here is meant to connote a small distance between two rollers, which does not exceed the safety regulations laid down by the law or professional associations in the respective technical field or the respective country or which suggests safety precautions based on other considerations. In Germany, special safety precautions apply in this connection that necessitate special safety measures if the roller nip exceeds 120 mm. This is intended, for example, to prevent operating personnel from getting their limbs crushed in the roller nip or to mitigate the consequences of such crushing hazards.
Most roller nips in the machines cited above by way of example are characterized by a direct mechanical contact between the transported material (inter alia sheets or webs) and the two rollers. This is the case, for example, between the printing substrate and printing plates and impression cylinder during the operation of the machine. In a surface winder, such a situation exists particularly between the contact roller and the winding core during normal winding operation.
The transport of the webs or sheets through this roller nip is usually determined by the rotary motion of the webs delimiting the roller nip or the rotary motion of at least one subsequent roller. This at least one roller provides the torque required for transporting the material through the nip. Therefore, at least one of these rollers is driven by an electric drive.
Electric machines of a wide variety of designs are used for this purpose in printing machines, winders, and packaging machines. These electric machines include synchronous or asynchronous electric motors, and DC-operated electric motors are also common.
These electric drives 30 can be provided with power controllers that make available the appropriate form of electric power for the motor.
As mentioned above, the roller nips are subject to observation for safety reasons. For this purpose, crush barriers and/or light barriers, which can generate a “Stop signal” for the drives of the relevant rollers, are often used in front of the roller nips.
Another option for preventing crushing hazards in roller nips while simultaneously ensuring the maximum possible accessibility of the roller nip consists in reasonably limiting the torque of the at least one roller, which provides the torque for transporting the webs or sheets through the roller nip. This can be accomplished by limiting—usually controlling—the torque-generating current. In this manner, it could be possible to operate the roller at a torque that does not exceed hazardous levels. However, an “Emergency stop” is also possible as a result of an increase in torque. Such an increase in torque can be triggered by a foreign body—such as a hand—in the roller nip.
Commercially available power controllers, which also include frequency inverters for three-phase motors or alternating current motors, also have the option of measuring the current at one of their outputs. So-called “shunts,” thus backup resistors, are often provided for this purpose. This measurement can form the basis of the torque control or emergency stop.
However, it has been seen that a current measurement using only one measuring system 50 involves safety risks. It may happen that such a measuring system measures inaccurately or does not measure at all and thus signals excessively low actual values of current to the power controller or control device. Consequently, the current regulator supplies an excessively high torque-generating current to the related drive. The drive would thus be able to generate an excessively high torque and the entire safety device would be worthless.